The present invention relates to 18F-labeled thia fatty acids, and to methods of making and using the same, in particular, to methods of using such fatty acids as a tracer compound in positron emission tomography (PET).
A radioiodinated 4-thia fatty acid analog has been previously reported (Gildehaus et al, J Nucl Med 38:124P, 1997, abstract). 18F-labeled fatty acids are known generally from U.S. Pat. No. 4,323,547 (incorporated hereinto by reference) as useful in PET studies of myocardial metabolism. More recently, 18F-labeled 6-thia fatty acid (14F6THA) has been synthesized and evaluated (DeGrado, J Lab Compd Radiopharm 24:989-995,1991; DeGrado et al, J Nucl Med 32:1888-1896, 1991, each incorporated hereinto fully by reference). Although 14F6THA tracks beta-oxidation of palmitate in a number of conditions, it was found to be insensitive to inhibition of beta-oxidation in myocardium in conditions of hypoxia with normal blood flow. Retention of tracer in hypoxic myocardium likely reflects retention of metabolic intermediates that precede beta-oxidation (long chain acyl-CoA, acyl-carnitine, and/or esterified lipids).
The present invention is based on the discovery that sulfur heteroatom substitution at the C4 position, instead of the C6 position, of 18F-labeled fatty acids yields a tracer that is retained in proportion to the beta-oxidation rates in normoxic and hypoxic mammalian tissue, particularly normoxic and hypoxic myocardium. Most preferably, the invention is embodied in an [18F]fluoro-4-thia-fatty acid having a chain length of between 8 to 20 carbon atoms, and may be saturated or at least partially unsaturated (i.e., contain one or more double bonds).
The 18F-labeled 4-thia fatty acids of this invention find particular utility in the noninvasive assessment of regional beta-oxidation rates using PET techniques which may allow early detection of abnormalities in the myocardium that might presage irreversible tissue injury.
Although not wishing to be bound to any particular theory, it is surmised that the 4-thia intermediates that precede beta-oxidation are poorly retained in the myocardium, possibly due to facile hydrolysis of the CoA and/or carnitine esters.
The [18F]fluoro-4-thia-fatty acids according to this invention are most conveniently synthesized by subjecting a hydrolyzable ester precursor of a 4-thia-fatty acid having a readily substitutable group at the terminal carbon or an odd-numbered carbon from the terminal carbon to 18F substitution conditions. Thereafter, the 18F-substituted hydrolyzable ester precursor of the 4-thia fatty acid may be subjected to hydrolysis conditions to form the corresponding [18F]fluoro-4-thia-fatty acid. Most preferably, the readily substitutable group is selected from bromo, iodo, tosylate, benzenesulfonylate and the like, while the group that makes the precursor readily hydrolyzable may be benzyl, methyl and the like. By way of example, methyl 16-bromo-4-thia-hexadecanoate is a synthetic precursor of 16-[18F]fluoro-4-thia-hexadecanoic acid. Bromo and iodo esters are preferable since their respective acids are easily separated from the radioactive product fatty acid.
The precursor is synthesized by conventional organic synthesis techniques. For example, methyl 16-bromo-4-thia-hexadecanoate is synthesized by reaction of methyl 3-mercaptopropionate with 1,12-dibromododecane in acetonitrile in the presence of potassium carbonate. The product ester is separated from reactants and other reaction products by silica gel liquid chromatography (hexane/ether 3:1, Rf=0.6). Labeling at the xcfx89-3 position requires two additional synthetic steps preceding the condensation reaction with methyl 3-mercaptopropionate, namely oxidation of xcfx89-bromo-(1)alcohol to the (xcfx89-bromo(1)aldehyde in dichloromethane using pyridinium chlorochromate followed by reaction of the aldehyde with propyl magnesium chloride in ether. The resultant (xcfx89-3)-alcohol is then condensed with methyl 3-mercaptopropionate to yield the hydroxyester. The hydroxyester is converted to the corresponding tosyloxyester by reaction with Ts-Cl in pyridine. Finally, the tosyloxyester is converted to the bromide, for example, by reaction with LiBr in acetone. Liquid chromatography is used at each step to isolate the products. 18F labeling may then conventionally be carried out as described in the literature cited above.
A further understanding of this invention will be obtained from the following non-limiting Examples.